A major problem associated with electroerosive machining is evacuation of the products of erosion. Efficient evacuation of the products of erosion facilitates producing holes with required depth and diameter. It also makes it possible to obtain shaped profiles of parts machined to a required precision at one set-up, that is without any pre-treatment or after-finishing. The rate and accuracy of machining further depend on the manner in which the energy released from the working end of the electrode tool is concentrated, and the method of machining selected.
There is known a method and device for evacuating the products of erosion and gas bubbles from an interelectrode gap (cf., Patent of West Germany No. 1,139,359; IPC B 23 P, published 1958) in which an electrode for making holes is fashioned as a cylinder having an inclined passage in its body for evacuating the products of erosion. The entire device, including the workpiece being machined, the electrode and the dielectric fluid, is accommodated in a sealed bath with an overpressure acting to force the fluid to move in the workpiece-electrode gap, entraining the products of erosion, and discharging them through the passage in the electrode.
However, this construction of the electrode fails to enable the machining of deep holes of small diameter. The reason for the failure is in that the considerable surface area of the electrode, as compared with the inlet area of the passage, prevents efficient evacuation of particles from this gap. In addition, the depth of holes thus drilled is never greater than the projection of the passage in the electrode onto the vertical centerline thereof.
These disadvantages are partially obviated by a French Pat. No. 1,178,722 IPC B 23 P, published 1957 and entitled "EDM Device and Method" in which, for removing the products of erosion, the electrode has the form of a hollow tube in which a dielectric fluid is supplied under pressure to escape with the products of erosion to a bath through an electrode-workpiece gap. An oscillatory motion is imparted to the workpiece with an amplitude causing cavitation in the dielectric fluid, this motion acting to discharge the products of erosion. The workpiece is simultaneously rotated.
However, this construction of the electrode fails to allow the drilling of holes at high magnitudes of discharge currents accompanied by the formation of large-size particle fraction of the products of erosion.
An electric pulse discharge causes a vapour fraction to appear and pitting to take place due to thermoresilient stresses to which the electrode is subjected. The particles of coarse fraction become larger in size as the power of the discharge grows and the duration of the pulse shortens. Therefore, an increase in the discharge current results in the coarse fraction failing to escape from the discharge gap to short circuit it and lead to the formation, on the end of the electrode, of growths of the products of erosion. Therefore, weak currents with rather sloping edges are necessary for operating such and electrode, which affects the efficiency of the electroerosive process, the reliability of the apparatus and the quality of machining.
In addition, the above methods and apparatus fail to produce deep holes of smaller diameter since the hole of the hollow electrode must be large enough so dielectric fluid can pass. The above method and apparatus also fail to produce holes of substantial diameter since in the course of electroerosive machining the material of the workpiece in proximity to the walls of the electrode at a distance of 1 to 1.5 mm is removed therefrom, whereas a projection tends to be formed in the center of the hole being drilled.
There is also known a device for electroerosive machining (cf., U.S. Pat. No. 2,718,581, Cl. 219-15, published 1955) in which inside a substantially hollow electrode there is symmetrically disposed another hollow electrode at one level with the first electrode. The second electrode acts to partially remove the core, while leaving in the center a non-removed portion.
At a certain depth of the hole being drilled the products of erosion fail to escape from the interior of the inner cylindrical electrode, whereby a further drilling becomes impossible. The products of erosion are evacuated with the dielectric fluid through the gap between the walls of the outer and inner electrodes.
However, the aforedescribed device suffers from the disadvantage that it enables the drilling of holes of only small diameter and depth. The device also suffers from insufficient reliability.
A device and method which bear the closest resemblance to those to be disclosed in the present specification are taught in French Pat. No. 2,097,709, IPC B 23 P 1/00, published 1971 and entitled "Electrode for Electroerosive Machining, Method of Using Thereof, and Device for Carrying out the Method". The device has an electrode tool provided with a central electrode enclosed by a shell, this central electrode being fashioned as a spirally wound rod having a rectangular or triangular cross-section and arranged at the level with the shell.
The central electrode is spirally wound so as to form, between the electrode and the inner surface of the shell, substantially spiral or helicoidal passages not extending beyond the longitudinal centerline of the electrode tool.
However, inherent in the above construction is a disadvantage in that the electrode tool ensures hole drilling to a low depth at a rather low efficiency of the electroerosive machining process. This disadvantage is accounted for by the fact that in the area of currents of considerable magnitude the large-size particle fraction fails to separate from the central electrode and is therefore welded thereto due to the small interelectrode gap. On the other hand, it is impossible to concentrate the liberated energy within a small portion of the electrode, which affects the efficiency of the electroerosive machining process, since the amount of the products of erosion increases with the growth in the density of the energy released. In addition, the spirally wound central electrode accommodated inside the shell offers resistance to the ascending flow of liquid carrying the products of erosion. This in turn results in deposition of the products of erosion on the faces of the spirally wound central electrode, build-up of these products thereon and clogging of the passage in the electrode tool, thereby making the device less reliable. In addition, the above construction of the electrode tool makes it impossible to drill holes of small dimensions due to the fact that the spiral shape of the central electrode has considerable cross-sectional dimensions.
There is further known an electroerosive machining method (cf., U.S. Pat. No. 2,902,584 Cl. 219-69) in which an electrode tool is advanced toward the workpiece being machined while the working end of the electrode tool is shifted for at least one combined motion in a direction different from the initial advance feed of the electrode tool. The working end of the electrode tool is displaced linearly toward the workpiece, and thereafter, for obtaining shaped holes, it is rotated about a center somewhat offset relative to the center of the working end of the electrode tool at a predetermined angle to displace in succession the working end of the electrode tool. This is followed by again rotating the working end of the electrode tool about the same center at a second predetermined angle. All these stages are repeated a required number of times.
The method, however, fails to ensure the production of deep holes of small diameter. It is also impossible to obtain shaped holes having acute or reverse angles, since the working end of the electrode tool is returned to the initial position after the first stage of the process has been completed without machining the inner surface of the workpiece.
The configuration of the hole being machined depends on the shape of the electrode tool; the electrode tool should preferably have a cross-section substantially identical to the configuration of the hole throughout its length. The electrode must move freely in the passage.